This invention relates to conductive transparent layers which have an indium oxide base doped with tin ("ITO"), and in particular, to those layers obtained by cathode sputtering on a glass substrate.
In many applications of transparent conductive layers as for instance in the case of an electrically heated windshield, it is desirable that a layer with a given thickness (which is usually determined for optical reasons) have the highest electrical efficiency. The resistivity of the layer is a good measure of this efficiency. The lower the resistivity, the higher the efficiency.
Of the methods proposed for producing transparent conductive layers with an indium oxide base doped with tin ("ITO"), the most common are vacuum techniques, one such technique being reactive cathode sputtering from a metal target composed of an alloy of indium and tin. This technique is similar to the method described in U.S. Pat. No. 4,336,119 which is incorporated herein by reference. However, this method achieves inadequate results because the layers that are obtained are neither very transparent nor very conductive.
A solution proposed in U.S. Pat. No. 4,512,864, which teaches heating the substrate before deposition of the layer, significantly improves matters and makes it possible to obtain good transparencies as well as good conductivities. However, on an industrial scale, there are considerable technical difficulties in introducing hot glass into a unit under vacuum and maintaining its temperature for the period of time needed to deposit a thick layer.
The same result may be achieved by (1) cathode sputtering on a cold glass support followed by (2) heat treatments. For example, CA Patent 566,773 (or GB 718,189) which is incorporated herein by reference proposes that ITO be deposited under substoichiometric conditions in oxygen and then the light transmission of the layer be increased by removing the glass support and deposited layer from the vacuum unit and raising their temperature to 350.degree. C. for five minutes. This process generally results in a transparent layer, but one that is not very conductive. Therefore, a second heat treatment in reducing atmosphere has been proposed in CA Patent 1,045,918 (or GB 1,498,142) to improve conductivity.
U.S. Pat. No. 4,490,227 also describes a similar process. In this process, a layer of ITO that is deposited by reactive cathode sputtering is subjected to the bending of the support glass, without cracking, and still retains minimum final optical properties. Since the general transmission conditions of this process are defined independently of the thickness of the layer, the intrinsic qualities of the layer are not taken into account.
Despite these developments it still is quite difficult to obtain conductive coatings of sufficiently low resistivity in an industrial scale process.